Abstract. Overshooting deep convection that penetrates into the Tropical Tropopause Layer (TTL) is
thought to have an important role in regulating the water vapour content of this
region. Yet, the net effect of such convection and the dominant mechanisms remain
unclear. This study uses two idealised three-dimensional cloud-resolving model simulations
to examine the influence of overshooting convection on water vapour when it penetrates into
two different TTL environments, one supersaturated and the other subsaturated with respect
to ice. These simulations show that the overshooting convection plays a direct role in
driving the ambient environment towards ice saturation through either net moistening
(subsaturated TTL) or net dehydration (supersaturated TTL). Moreover, in these cases the
extent of dehydration in supersaturated conditions is greater than the moistening in
subsaturated conditions. With the aid of modelled passive tracers, the relative roles of
transport, mixing and ice microphysics are assessed; ultimately, ice sublimation and
scavenging processes play the most important role in defining the different TTL relative
humidity tendencies. In addition, significant moistening in both cases is modelled well into
the subsaturated tropical lower stratosphere (up to 450 K), even though the overshooting
turrets only reach approximately 420 K. It is shown that this moistening is the result of
jumping cirrus, which is induced by the localised upward transport and mixing of
TTL air following the collapse of the overshooting turret.